Outline Of Todays Discussion 1 The Human Eye

Outline Of Today’s Discussion 1. The Human Eye: Phototransduction 2. Introduction To Retinal Ganglion Cells

Part 1 The Human Eye: Phototransduction

The Eye & Phototransduction 1. Embryologically, the retina originates from the same tissue that spawns all neurons. 2. Like other brain cells, cells in the retina cannot be repaired. 3. Most importantly, the retina is the location of phototransduction. 4. Did you know that the photoreceptors “face the wrong way”? ….

The Eye & Phototransduction Light enters from here. Anterior Posterior Photoreceptors face away.

The Eye & Phototransduction 1. So, this is the first instance of “something backwards” in our retina: Photoreceptors face away from the incoming light! 2. There are two categories of photoreceptor in the human eye: Rods, and cones.

The Eye & Phototransduction 1. Rods out number cones by approximately 100 million to 5 million, in each eye. 2. The most central part of the retina contains only cones. Cone density decreases with eccentricity, and in the far periphery, only rods are found. 3. Lumping rods and cones together, photoreceptor density (overall) decreases with eccentricity. So, vision is less sharp in the periphery.

The Eye & Phototransduction • For vertebrates, light increments lead to hyperpolarization, and light decrements lead to depolarization: Invertabrates, vice-versa. • So, we have another instance of “something backwards” in our retina: More light, less transmitter release.

The Eye & Phototransduction • Opsin in rods (rhodopsin) responds best to 500 nm wavelengths. • The three different cone-opsins respond best to 440, 530, and 560 nm wavelengths. • But, at the point of photo-transduction, all wavelength information is lost. This is the principle of univariance.

The Eye & Phototransduction • Today’s “Top Three” List • The top three reasons why it is VULGAR to refer to human cones as ‘red’, ‘green’, and ‘blue’. • The number 3 reason is….

Part 2: The Eye & Phototransduction 3. Newton said so!

“The Facts Of Light” “The rays are NOT colored. ” Sir Isaac Newton

“The Facts Of Light” “The red’s in the head. ” Billy Wooten

The Eye & Phototransduction 3. Newton said so! 2. The Peaks don’t match perception!

The Eye & Phototransduction 440 nm looks violet, not blue! 530 nm looks green…OK…but, 560 nm looks yellow, not red!

The Eye & Phototransduction 3. Because Newton said so! 2. The Peaks don’t match perception! 1. The Principle of Univariance!

The Eye & Phototransduction All wavelength information is lost at photo-transduciton. Any cone absorb any wavelength.

The Eye & Phototransduction So, no vulgarity, please.

Learning Check • Be able to describe three types of refractive errors, and the lenses used to correct them. • Be able to identify three reasons why it is inaccurate (“Vulgar”) to refer to our three cone types as red, green, and blue. • Be able to find your blind spot (each eye). That is, be able to “hide” your thumb in your blind-spot.

Part 2 Introduction to Retinal Ganglion Cells

Intro to Retinal Ganglion Cells Here’s a ‘big picture’ of the eye. Note, that the retina is at the posterior pole.

Intro to Retinal Ganglion Cells 1. The retina consists of three layers: Photoreceptors; Middle-layer (horizontal, bipolar, amacrine); Ganglion cells. 2. Let’s zoom in to see how these arranged…

Intro to Retinal Ganglion Cells Light enters from here. Anterior Posterior Photoreceptors face away.

Intro to Retinal Ganglion Cells 1. The ganglion cells, especially, are involved in extracting meaningful visual information. 2. The most meaningful information for an organism would be the identification of object boundaries and edges. 3. Typically, objects in the environment differ from each other and/or their surroundings with respect to light intensity. 4. The ganglion cells specialize in detecting those luminance boundaries, and reporting them to later portions of the visual system.

Intro to Retinal Ganglion Cells Location, Location! 1. Photoreceptors face away from the incoming light. They are the most posterior of the three retinal layers. 2. Within the middle layer, the horizontal cells are closest to the photoreceptors. The bipolar cells are next. The amacrine cells are furthest middle-layer cells from the photoreceptors. 3. The ganglion cells are the “last stop” before exiting the retina. 4. The retina is no thicker than a postage stamp!

Intro to Retinal Ganglion Cells All these cell types are in the retina. Anterior Posterior The retina is no thicker than a stamp!

Intro to Retinal Ganglion Cells A Memory Aid For Visual Anatomy: R. H. Bagols 1. 2. 3. 4. 5. 6. 7. 8. Receptors Horizontal Cells Bipolar Cells Amacrine Cells Ganglion Cells Optic Nerve Lateral Ganiculate Nucleus Striate Cortex (Primary Visual Cortex: “V 1”; “Area 17)

Intro to Retinal Ganglion Cells 1. Ganglion cells don’t respond directly to light! They respond to the output of middle-layer cells. 2. There approximately 1. 25 million ganglion cells per eye. 3. So, there is a nearly 100 -to-1 ‘data reduction’ within the retina. The summary specifies edges and boundaries. 4. By analogy, ganglion cells would summarize a 750 -1, 000 word essay into 7. 5 to 10 words.

Intro to Retinal Ganglion Cells Receptive Fields 1. Electrophysiological recordings are conducted to determine a ganglion cells receptive field. 2. The receptive field is the area of the photoreceptor mosaic to which a ganglion cell responds. 3. Equivalently, when the eye is immobilized, the receptive field corresponds to a region of space, since light travels in straight lines. 4. Receptive-field size is smallest in the fovea (the center of the macula). The smaller the receptive-field size, the sharper the vision.

Intro to Retinal Ganglion Cells Center-Surround Antagonism 1. Ganglion cells exhibit center-surround antagonism: Stimuli that excite the center, suppress the surround, and vice versa. 2. Center-surround antagonism within a ganglion cell is a form of lateral inhibition. 3. Lateral inhibition - Antagonistic neural interaction between adjacent regions of a sensory surface, such as the retina.

Intro to Retinal Ganglion Cells Center-Surround Antagonism • For ganglion cells, there are two types of centersurround antagonistic arrangements: “On. Center” and “Off-Center”. • “On-Center” and “Off-Center” cells are equally numerous, and their axons remain segregated through to the next stage in the visual pathway.

“On-Center” Ganglion Cell - - - ++ ++ - + + + - Responds maximally to light increments in the center, and light decrements in the surround.

“On-Center” Cells Love Stimuli like this. - - - ++ ++ - + + + -

“On-Center” Cells Hate Stimuli like this. - - - ++ ++ - + + + -

“Off-Center” Ganglion Cell + ---- - + + + + Responds maximally to light decrements in the center, and light increments in the surround.

“Off-Center” Cells Love Stimuli like this. + + -- + -+ - -- + +

“Off-Center” Cells Hate Stimuli like this. + + -- + -+ - -- + +

Ganglion cells have no orientation preference.

Intro to Retinal Ganglion Cells The visual system responds to CONTRAST!!!

Intro to Retinal Ganglion Cells Michelson Contrast = (Lmax - Lmin) _______ (Lmax + Lmin)

High Contrast Stimuli

Low Contrast Stimuli

No-Contrast Stimuli (No Difference Between Center and Surround)